To test out the effects of several nucleosides and other chemicals, such as neomycin upon a 'transmissible entity' that originated by or became measurable following the intraperitoneal injection of a specially prepared liver extract. The criteria for the presence of the 'transmissible entity' are the progressive or retrogressive growth rates of spontaneous tumors of mammary gland origin--the same technique that was used in the determination of sudden changes in the transplantable neoplasm reactivity -- which discovery was one of the foundation stones for the study of histocompatibility used in the genetic analysis of susceptibility and resistance to transplanted tumors.